1. Field of the Invention
The present invention is related to the technical field of microwave antennas and may be used, for example, with reconfigurable antennas for use on artificial satellites or space stations or in ground radar systems. In particular, the invention is directed to a double-reflector microwave antenna belonging to the family of Gregorian antennas which, through the rotation of its sub-reflector and/or the axial movement of the sub-reflector or a main reflector, achieves the rotation of an elliptical beam without substantially any variation to the beam width and polarization and/or reconfigurability of the beam into a circular, expanded ellipse (also referred to herein as "zoom effect") or intermediate ellipse between the original beam and the circular variation of the beam shape. Moreover, by using another sub-reflector profile, the circular beam may be widened (zoom) into another circular beam.
2. Description of the Related Art
Moderate reconfigurability requirements of future antenna systems include the following functions:
(a) re-pointing of the beam; PA1 (b) turning the elliptical beam without rotation of polarization; PA1 (c) zooming of a circular or elliptical beam, that is, broadening of the beam coverage with substantially no variation of the ratio between the ellipse axes and the area x gain product; and PA1 (d) converting a circular beam into an elliptical beam, and vice versa, with substantially no variation of the area x gain product. PA1 (I) minimal increase in the dimensions and mass of the antenna; PA1 (II) substantially no movement of large masses; PA1 (III) substantially no movement of the illuminators, which is undesirable in the presence of relatively high power levels; PA1 (IV) substantially no movement of parts internal to the illuminators, which may potentially generate deleterious intermodulation products; PA1 (V) maximum reliability and simplicity and a minimum number of actuators; and PA1 (VI) minimum sensitivity to alignment errors and thermal excursions.
Of these four functions, only function (a) is generally available for Ku band communication satellite antennas. Functions (b), (c) and (d) are highly desirable and, together with function (a) (and combinations thereof), in principle set only by the capacity of the type of antenna considered to not degrade the quality of service as a consequence of the greater flexibility so assured. The partial implementation of functions (b), (c) and (d), however, should also satisfy the following criteria and result in only:
Thus, it is desirable to develop an antenna configuration capable of achieving the reconfigurability functions by acting upon system optics and, as far as practicable, avoid substantially all movement of the illumination system or of large masses.
Antennas capable of electrical performance adequate for present requirements for satellite communications are classified as double reflector Gregorian optics. These optics provide relatively high coverage efficiency, relatively low side lobes and, when particular geometric relations are met, relatively high polarization purity with a size and mass suitable for installation on board satellites, as for example Intelsat VIII satellites.
The general geometry of the Gregorian optics family is shown in FIG. 4. The Gregorian optics include a main reflector 3, a sub-reflector 2 and a suitable feed 1. The Gregorian optics comprise the same elements found in the present invention; however, the elements of the present invention are distinguishable in their movement and surface profiles. Typically, the design of classical Gregorian antennas begins from the canonical surfaces. In FIG. 4, sub-reflector 2 is ellipsoidal having two foci 21, 22 and main reflector 3 is parabolic having a focus 8 which coincides with the first focus 21 of the sub-reflector. These surfaces provide extremely low cross-polarization levels when the geometric requirement for maximum purity shown in FIG. 4 is met. This condition is met when eccentricity e of the ellipsoid of sub-reflector 2 satisfies the geometric relation of angles .beta..sub.f and .beta..sub.s shown in FIG. 4. In this figure, .beta..sub.f is the angle between the symmetry axis 9 of the illuminator 1, whose phase center 7 coincides with a second focus 22 of the ellipsoidal sub-reflector 2, and propagation axis Z. Angle .beta..sub.s is the angle between symmetry axis 9 and a rotation axis of symmetry 10 of the sub-reflector surface which intersects through both foci 21, 22 of the ellipsoid. Sub-reflector 2 of FIG. 4 is an ellipsoid obtained by revolution about axis 10, while the optics of the present invention have a sub-reflector surface which cannot be obtained by revolution about the axis crossing points 7 and 8.
The optical system shown in FIG. 4 may be used to generate a circular beam. In addition, the standard optics of FIG. 4 are commonly used to generate an elliptical beam by shaping the sub-reflector surface 2 and/or the main reflector 3 numerically and accepting the electrical degradations in terms of polarization purity which derive from this upset system. These degradations are generally acceptable since the deviations introduced onto the surfaces are relatively small. This optical system clearly cannot, however, provide a rotation of the elliptical beam by turning the sub-reflector.
Presently, there exist no known solutions exist which allow the reconfiguration of the beam in terms of rotation and/or reconfiguration and/or widening (zoom) of the contour of the beam on such types of antennas using a single feed. The only function available today on such antennas is beam repainting, a function that is normally performed through a system of biaxial actuators within a cone of approximately .+-.11.degree. which represents the useful field of view of the Earth from a geostationary orbiting satellite.
Another class and type of antennas, dual-gridded reflector type optical systems, are commonly used to obtain shaped antenna beams when a reconfigurable contour and relatively high polarization purity is required. This type of system includes two feed arrays located in the focal plane of the optical system, a rear reflector and a front reflector. As shown in FIGS. 27a through 27c, the front reflector is implemented by application of linear metal strips onto the dielectric surface of the front shell and the back reflector may be either solid or gridded with strips arranged orthogonal to those of the front reflector. In particular, FIGS. 27a, 27b and 27c show front, top and side elevational views, respectively, of the dual-gridded reflector type optical system. As shown in these figures, a group of feeds 1 provides polarization of the electrical field along the X axis and a corresponding group of illuminators 1' provides polarization along the Y axis. The optical system also includes a gridded front reflector 3 that is sensitive to X polarization and a rear reflector 3', which may be either solid or gridded, that is sensitive to Y polarization.
The characteristics of this optical system are such that each reflector operates in single polarization mode and benefits by the space filtering effect of the other reflector on the cross-polarization components that would otherwise be radiated over the service coverage. The radiating elements are typically excited by a beam forming network which includes microwave components capable of changing the excitation of the radiating elements placed in the focal plane through power dividers and/or phase shifters. This technique is based on reconfigurable feed arrays that belong to another class of antenna families. The present invention, however, is directed to reconfigurable single feed antennas that are extremely simple, lightweight and capable of exploiting the optics degree of freedom to improve electrical performance relative to multifeed antennas having the same main reflector aperture, a functionality not provided by the prior art.